In some system designs, a fixed frequency is required. For example, a fixed frequency is needed to reduce electromagnetic interference (EMI) in some portable devices. One prior art solution is to use a clock circuit, a ramp compensation circuit and an amplifier to form a closed loop to obtain the fixed frequency. The drawback of this conventional solution includes, but is not limited to, a slow regulation speed.
Another existing solution is to use an adaptive constant on time control with an input voltage feed forward. This solution can almost achieve a fixed frequency. However, it regulates an inductor valley current so that the accuracy of the LED current regulation is typically not good. In addition, a load step down transient may have a big overshoot due to the constant on time.
FIG. 1 shows a circuit 10 illustrating a conventional hysteretic control circuit 100 for an LED driver with a buck converter. As shown in FIG. 1, an input voltage Vin is provided to a first terminal of a high side switch Q1 whose second terminal is electrically coupled to a first terminal of a low side switch Q2. A second terminal of the low side switch Q2 is electrically coupled to the ground. An inductor L is electrically coupled between a node SW formed by the second terminal of the high side switch Q1 and the first terminal of the low side switch Q2 and an output voltage port which provides a regulated output voltage Vo to an LED string. A capacitor Co is electrically coupled between a first terminal and a second terminal of the LED string. A sensing resistor Rsensed is electrically coupled between the second terminal of the LED string and the ground.
The hysteretic control circuit 100 comprises a fixed hysteretic width production circuit 101, a comparator CMP, an inverter INV and a hysteretic band voltage generating circuit 102. As shown in FIG. 1, the inductor current IL is sensed by the sensing resistor Rsensed across which the voltage drop acts as a sensing voltage Vs to be compared by the comparator CMP with a hysteretic band voltage which comprises a high hysteretic band voltage V_h by adding half of a hysteretic width ΔV generated by the fixed hysteretic width production circuit 101 with a reference voltage Vref and a low hysteretic band voltage V_l by subtracting half of the hysteretic width ΔV from the reference voltage Vref. When Vs is lower than V_l (Vs<V_l), the comparator CMP outputs a high level to turn on Q1 and to turn off Q2 with a low level which is generated by inverting the high level with the inverter INV. Accordingly, the inductor current IL and the sensing voltage Vs start to increase. When Vs increases to such a point that it is higher than V_h (Vs>V_h), the comparator CMP outputs a low level to turn off Q1 and meanwhile to turn on Q2 through the inverter INV. Accordingly, the inductor current IL and the sensing voltage Vs start to decrease. When Vs decreases to be lower than V_l (Vs<V_l) again, the comparator CMP outputs a high level to turn on Q1 and to turn off Q2 through the inverter INV. A new control cycle begins.
The on time T1 and the off time T2 of the high side switch Q1 are determined by the hysteretic width ΔV, the inductor L, the input voltage Vin, the output voltage Vo and the sensing resistor Rsensed, following the equations below:ΔI=ΔV/Rsensed  (1)T1=(L×ΔI)/(Vin−Vo)  (2)T2=(L×ΔI)/Vo  (3)Thus, the switching period Ts of the switches can be written as:Ts=T1+T2=(L×ΔV×Vin)/(Rsensed×(Vin−Vo)×Vo)  (4)In a particular application, the sensing resistor Rsensed is determined by a setting LED current, and the inductor L is determined by the inductor current ripple and the output voltage Vo is expected to be constant. So the switching period Ts is dependent of the input voltage Vin and the hysteretic width ΔV.
For the conventional hysteretic control circuit for an LED driver with a buck converter shown in FIG. 1, the hysteretic width ΔV is a fixed value generated by the fixed hysteretic width production circuit 101, so the switching period Ts is only dependent of the input voltage Vin. When Vin changes, Ts changes accordingly. That is to say, the switching frequency Fs=1/Ts changes in response to the input voltage Vin. FIG. 2 shows a waveform diagram illustrating examples of signals in the conventional hysteretic control circuit shown in FIG. 1. The signals from top to bottom are followed by the sensing voltage Vs, the voltage VSW at the node SW and the inductor current IL. Accordingly, improved hysteretic control circuits and methods thereof for LED drivers are needed.